1. Technical Field
The present invention relates to a mask assembly and a method of fabricating the same and, more particularly, to a mask assembly and a method of fabricating the same in which, when the mask assembly is fabricated by tensile welding a pattern mask to a mask frame, a tensile welding portion of the pattern mask welded to the mask frame is structurally changed to facilitate the welding of the pattern mask to the mask frame and fine alignment of a pattern of the pattern mask, thereby minimizing a stain failure rate due to the mask assembly.
2. Related Art
A flat panel display device (FPD) has been employed as a display device which supersedes a cathode ray tube (CRT) display device because the FPD is lightweight and thin. Typical examples of the FPD are a liquid crystal display (LCD) and an organic light emitting diode (OLED) display device. In comparison with the LCD, the OLED display device has a higher luminance and a wider viewing angle and can be made ultrathin because the OLED display device needs no backlight.
The OLED display device includes an organic thin layer having at least one organic emission layer (organic EML), which is interposed between an anode and a cathode. Thus, when a voltage is applied between the anode and the cathode, electrons injected through the cathode and holes injected through the anode recombine in the organic EML to produce excitons. As a result, light with a specific wavelength is generated due to the energy of the excitons.
During fabrication of the OLED display device, the cathode, the anode and the organic thin layer may be selectively formed on a substrate formed of glass, stainless steel or synthetic resin by means of a photolithography method or a deposition method using a mask assembly including a pattern with a plurality of slits. During the photolithography method, after coating photoresist on a portion of the substrate, the photoresist may be delaminated or etched using a wet or dry etching process. In this case, moisture may diffuse into the substrate during the etching process. Accordingly, a material layer, for example, the organic thin layer, which may be degraded due to moisture, may be typically formed using the deposition method using the mask assembly.
In order to display full color, the OLED display device includes an OLED having red(R), green (G) and blue (B) organic EMLs. In detail, a mask pattern having a plurality of openings is aligned on the substrate where the OLED having the R, G and B organic EMLs will be deposited. Thereafter, the R, G and B organic EMLs are deposited through the openings of the mask pattern so as to have desired shapes of patterns. As a result, the OLED having the R, G and B organic EMLs is formed on a predetermined region of the substrate. Accordingly, it is very important to precisely align the mask pattern on the substrate.
In order to form the mask pattern with higher positional precision, a plurality of stick pattern masks, patterns of which are arranged in a first direction may be tensile-welded to a mask frame. However, it is necessary to prevent formation of a gap between the stick pattern masks, and aligning and welding the stick pattern masks are not easy due to the movement of the stick pattern masks.
In addition, in order to solve the above-described problem of the stick pattern masks, a pattern mask having a pattern portion with patterns arranged in a matrix may be tensile-welded to a mask frame. However, when corrective welding is performed in order to precisely align patterns arranged in each row or column, the positions of patterns disposed in neighboring rows or columns are readjusted, thereby lowering positional precision of the mask assembly and increasing a stain failure rate. A stain failure occurs when the mask patterns in a mask assembly are not fully aligned and a gap exists between neighboring mask patterns, causing undesirable deposition portions through the gap during a subsequent deposition process, and thus resulting in a stain on a substrate. The term “stain failure rate” indicates the likelihood of a stain failure when a mask assembly is used for deposition.